1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a handover method in a mobile communication system used in multiple radio zones.
2. Description of the Related Art
In a conventional mobile communication system that provides services with mobile unite (mobile stations) such as automobile or portable telephones, a system is employed whereby the entire servicing area of the mobile communication system is divided into small zones which are covered by a plurality of radio zones. The radio zone here means a range of area in which communication is made between respective radio base stations and mobile units through radio channels, and there is one radio base station in one radio zone and each mobile unit can freely move within this radio zone.
Also, each radio base station can utilize a plurality of radio channels and, based on the condition of radiowave propagation between the radio base station and the mobile unit concerned in communication, the radio channel used is decided. Further, when, in the course of receiving the services from the mobile communication system, the mobile unit experiences a deterioration of radiowave propagation environments of the radio channel being used, it is possible for the mobile unit to change the channel to another channel. This change-over of channels during the communication is called "handover". The device which controls the radio base stations and carries out exchanges with fixed networks (public telephone networks) is the mobile switching center.
Where the small zone system as explained above is adopted, it is necessary that the current location of the mobile unit be registered in order to receive calls addressed to the traveling mobile unit. Various location registration systems have been proposed, and one of them has been disclosed in Japanese Patent Application Kokai Publication No. Hei 5-3581.
Two typical handover systems, according to a reference (S T S Chia, "The control of handover initiation in microcells", IEEE VTC '91, pp. 531-536, 1991), are a backward handover initiation via the existing radio channel and a forward handover initiation via the target radio channel.
In the conventional systems, as shown in a reference (Fumiaki Ishino at. al, "Mobile Communication Switching System for Voice/Nonvoice Services", IEEE GLOBECOM '91, pp. 485-1489, 1991), the existing radio base station and the target radio base station are multi-connected by the mobile switching centers during the handover in order to prevent the occurrence of a momentary disruption at the handover.
In the foregoing mobile communication system, as a means to effectively utilize radio frequencies, each of the radio zones is made small so that the same frequency may be repeatedly used in remote zones. Along with the making of zones smaller in this way, the handovers with which the mobile unit travels from one radio zone to another have become more frequent, resulting in a burden to the mobile switching center. Also, the multiple radio zone environment in which radio zones of varying sizes coexist necessitates the handover processing among the radio zones of different sizes. Thus, various means are being studied so as not to increase the volume of control processing of handovers.
A method for processing calls or communications has been disclosed in Japanese Patent Application Kokai Publication No. Hei 2-143725 in which, for selecting control channels in a multiple radio zone environment, the traveling speed of the mobile unit is estimated using the fading pitch of changes in the received signal level or the number of level crossings and, when the traveling speed of the mobile unit is high, the control channel of the large radio zone is selected while, when the traveling speed of the mobile unit is low, the control channel of the small radio zone is selected.
In the conventional technology, where the arrangement disclosed in the above Japanese Patent Application Kokai Publication No. Hei 2-143725 is applied to the handovers in the multiple radio zone environment, since the size of the handover target radio zone needed is selected based on the physical speed of the mobile unit, it requires a device which measures the fading pitch or the number of level crossings. Also, due to differences in the mobile characteristics such as the size of the radio zone in which the mobile unit currently exists and the ways in which the mobile unit travels, the mobile unit which travels faster does not necessarily traverse a larger number of boundaries of radio zones, hence a larger number of handovers. Furthermore, in the future, the multiple radio zone environment in which the radio zones of varying sizes coexist may change so as to cover a very small zone, smaller than that now contemplated or a very large zone, or a nation wide radio zone realized by use of satellite, in which case the selection of radio zones by the physical speeds of the mobile unit will become meaningless due to diversification of environment.